The invention relates to a measuring device for measuring the bioelectrical activity of the central nervous system in which a plurality of electrodes which can be placed on the patient are employed and in which the potential differences between the electrodes and a reference potential derived from the electrode potentials are formed.
In known measuring devices of this type, the measurement ensues either by means of a plurality of electrodes arranged according to an international standardization on the scalp (electroencephalograph EEG) or by means of a plurality of electrodes which are applied to the exposed cerebral cortex or to the meninx (electrocortigraphy, ECoG). The electrical activity of the nerve cells and of the surrounding medium is comprehended under the electrodes as corresponding potential changes. In both cases, amplifiers and registering devices are post-connected to the electrodes.
The known measuring processes which are carried out with such measuring devices are divided into bipolar and unipolar measuring processes as they are illustrated and described in the "Handbook of Electroencephalography", Elsevier Scientific Publishing Company, Amsterdam, 1974, Volume 3, Part 3, particularly in FIGS. 10 and 13 with the appertaining description. In a bipolar measuring process, differential voltages are conducted to the amplifier inputs which are realized in pairs between the electrodes. Thereby, each measured voltage is the difference between two electrode potentials. A selective covering of each localized electrode potential change does not ensue. Accordingly, it is difficult to precisely localize the cerebral bioelectrical activity. In a unipolar measuring process, differential voltages are comprehended between a plurality of electrodes and a respective reference point provided in common for these electrodes. This reference point can be a physical electrode or, for example, the midpoint of a resistance star which is connected to all electrodes with the same amount of resistance, possibly with the exception of those electrodes whose signals could falsify the measuring result because, for example, they are caused by means of muscle activity. In the majority of electrodes employed, it has up to now resulted by necessity that not all reference-forming electrodes are arranged neighboring the signal electrode, but that, rather, they were distributed over the entire scalp of the patient. For this reason, it was not possible to comprehend a potential which was a measure for the bioelectrical activity in the direct area of the signal electrode. On the contrary, only potential differences were measured which allowed of no precise conclusions concerning the location of the measured bioelectrical activity.
In the German AS No. 25 18269 (corresponds to the U.S. Pat. No. 4,084,583), a measuring process is described in which only such electrodes are used as the auxiliary electrodes for a signal electrode which neighbor the signal electrode. In the registration of a cerebral electrical activity with the assistance of surface electrodes on the scalp, however, a spreading effect is present in contrast to the registration of potential distributions in tissues which lie deeper, for example, on the cortex. There is a desire, however, to know the potentials prevailing on the cortex.